The instant invention relates to magnetic brush development devices in electrophotocopying machines, and more particularly to controlling the bias voltage on the magnetic brush development device and the charging of the photoconductor in a compact electrophotocopying machine utilizing a two-component developer material.
Plain paper electrophotocopying machines are well known devices that reproduce images of original documents on ordinary or plain paper. Such machines typically include a re-usable photoreceptor including a conductive substrate coated with a photoconductive material, a supply of plain paper copy sheets, and a plurality of processing stations for operating on the photoconductor and the copy sheets. In a typical sequential copying process, the photoconductor in uniformly charged and then exposed to an illuminated, original document. The areas of the photoconductor that are struck by the image of the indicia carried on the original retain their charge and the charge in the light-struck areas is dissipated. Thus a latent electrostatic image is formed on the photoconductive coating of the photoconductor. The latent image is then developed by contacting the photoconductor with electrostatically attractable, two component developer material which adheres to the charged areas of the photoconducting coating that make up the latent image. The toned image is next transferred to the copy sheet where the toner is fused onto the sheet to fix the image thereon.
Early models of plain paper copiers were large in size because they used either a relatively large radius drum for supporting a photoconductor or a belt or web type photoconductor that moved through an elongated path past the copy processing stations. Unfortunately, such machines are not practical for some users who have a limited amount of office space or modest copying requirements.
Accordingly, the size of plain paper copiers in many instances has been reduced in order to meet the spatial and economic requirements of the relatively low volume user who desires plain paper copies. However, these compact, plain paper copiers with unity magnification pose additional problems, one of which comes about when the length of the photoconductor (which may be reduced in order to provide a compact, plain paper copier) is less than the length of the longest document to be imaged plus the length required for the physical location of all the photocopying process elements other than the apparatus for transferring the latent image from the photoconductor to a copy sheet (typically the transfer corona). Although the magnetic brush is utilized for for both development of the latent image as well as cleaning of the untransferred toner there is not enough space along the periphery of the short photoconductor used in this compact copier to physically locate all the photocopying process elements other than the transfer apparatus. The result is that the photoconductor is required to rotate through two cycles for each desired copy of an original document. A typical plain paper copier employs corona devices for charging and discharging the photoconductor and for transferring the toned image on the photoconductor to the copy sheet. However, when the photoconductor is caused to pause in its movement in order to let the rest of the photocopier cycle synchronize to the photoconductor movement, the discharging corona can permanently damage the photoconductor. Accordingly, the power supply to the discharging corona is terminated when the clutch driving the photoconductor is disengaged from the photoconductor.
Some compact, plain paper copiers which function as described above, are provided with a common, internally generated source of voltage for the magnetic brush developing device bias voltage and the discharging corona because they are operated simultaneously, thereby allowing the cost of the machine to be reduced. However, when the common source of voltage to the discharging corona is terminated to prevent permanent damage to the photoconductor, the magnetic brush developing device bias voltage is also terminated, which results in a small band of undesirable background across the copy sheet. The instant invention provides an improvement to reduce the nonuniform photoconductor damage caused by the discharging corona and to maintain constant the bias voltage associated with the magnetic brush developing device to overcome the aforementioned undesirable background.